There is a need to detect mechanical vibrations caused by various sources including sound from a distance. One application includes the covert detection of voices in a room. A laser vibrometer or “laser microphone” is known (at least in theory) wherein a laser beam strikes a vibrating window due to people talking or a speaker operating in a room. The laser beam is split and a detector is responsive to both the split beam and the beam reflected off the window. Using interferometry, the sounds causing the surface (e.g., a window) to vibrate, can be reconstructed.
There are numerous problems associated with such an approach including the need for an active laser, accounting for differences in beam leg length, and the need for complex hardware, circuitry, and software.